Dielectric member for absorbing thermal expansion and contraction at electrical interfaces

ABSTRACT

The invention is directed to a dielectric member interposed between two electrical components which have different coefficients of thermal expansion (CTEs). The dielectric member has conductive traces for electrically connecting the electrical components. The traces may be joined by solder balls to a printed circuit board. The dielectric member may include reservoirs for locating the solder balls and receiving solder after reflow of the solder balls. Adhesive layers may be used for bonding the traces to the dielectric member. The dielectric member is made of a material having a selected CTE value which minimizes the CTE mismatch at the electrical interface and effectuates absorption of the thermal expansion and contraction of the system. Stresses induced by thermal expansion and contraction at the electrical interface are thereby reduced, preventing problems such as fractured solder joints.

FIELD OF THE INVENTION

The invention is directed towards a dielectric member for interpositionbetween a first electrical component, such as an electrical socket, anda second electrical component, such has a printed circuit board, whichhas a preselected coefficient of thermal expansion (CTE) that relievesexisting CTE mismatches between first and second electrical components.

BACKGROUND OF THE INVENTION

Interfaces between separate electrical components which are subjected tothermal cycling typically experience stresses caused by the differentrates of expansion and contraction of each electrical component. Forexample, a first electrical component may have a low CTE while thesecond electrical component has a relatively higher CTE, indicating agreater degree of thermal expansion and contraction. In particular,electrical connectors mounted to printed circuit boards virtually alwayshave higher CTE values than the printed circuit boards on which they aremounted. This CTE mismatch results in a relative motion between thefirst and second electrical components at their interface.

One arrangement which is particularly subjected to CTE mismatch is amicroprocessor housed in a socket and mounted on a printed circuitboard. In this arrangement, the components are subjected to extremethermal cycling. The microprocessor generates heat during operation thatis transferred to the electrical socket which houses the microprocessor.Because of the difference in base materials between the microprocessorand the electrical socket (the processor is typically made from aceramic or resin material while the electrical socket is molded from aninsulative plastic) a CTE mismatch is encountered at theprocessor/socket interface. The CTE mismatch at this interface istypically not problematic because there are no rigid points ofelectrical connection (e.g., solder joints) between the processor andthe socket. Therefore, the difference in thermal expansion andcontraction between the socket and the processor may be absorbed by therelatively tolerant electrical connections between the socket and theprocessor.

However, the electrical socket is typically soldered to a printedcircuit board in a through-hole or surface mount configuration whichrequires rigid and relatively inflexible solder joints. And, as with theprocessor and the socket, the printed circuit board is subjected tofairly extreme thermal cycling which is also transferred to theelectrical socket. Typical CTE values for printed circuit boardmaterials fall between the range of 12 and 18 ppm/° C., which indicatesrelatively little expansion and contraction when subjected to thermalcycling. On the other hand, a molded electrical socket manufactured froman insulative plastic material may have CTE values ranging fromapproximately 15 to 70 ppm/° C. These CTE values indicate that theprocessor socket will expand and contract at a greater rate than theprinted circuit board when subjected to thermal cycling. As a result,rigid electrical connections such as solder joints between the processorsocket and the printed circuit board are subjected to induced stresseswhich frequently cause solder joints to fracture thereby causingelectrical failure at the joint.

Efforts have been taken by electronics manufacturers to enhance orreinforce solder joints at the socket/pcb interface to prevent fractureand resulting electrical failure. However, these efforts too can produceunreliable results. For instance, it is difficult to ensure uniformsolder joints when a large array of electrical contacts is used. Thisproblem is frequently manifested in the occurrence of solder-wicking.Solder-wicking occurs when, by capillary action, solder flows along theelectrical contact and away from the desired point of electricalinterconnection. This results in a weaker, less reliable solder joint.

Accordingly it would be desirable to provide a way of accommodating orminimizing the effect of CTE mismatches between separate electricalcomponents such as a processor socket and a printed circuit board. Itwould also be desirable to improve the reliability of solder jointsbetween electrical components by improving their uniformity andinhibiting occurrences of solder-wicking.

SUMMARY OF THE INVENTION

In accordance with the objects of the present invention, a socket forreceiving a semi-conductor package is provided having a housing with aplurality of electrical contacts. A dielectric is provided having aplurality of first conductive sites exposed on a top surface of thedielectric and a plurality of second conductive sites exposed on abottom surface of the dielectric. The first and second conductive sitesare electrically interconnected. The plurality of electrical contactsare electrically connected to the first conductive sites while thesecond conductive sites are connected to an electrical component.

A dielectric member for interposition between a first electricalcomponent and a second electrical component is provided. A plurality ofelectrically conductive members are held within the dielectric memberhaving an exposed top surface and an exposed bottom surface forelectrical connection with the first electrical component and the secondelectrical component, respectively.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described in detailwith reference to the accompanying drawings, in which:

FIG. 1 is a partial cross-sectional side view of a prior artmicroprocessor socket mounted on a printed circuit board;

FIG. 2 is a partial cross-sectional side view of an embodiment of thepresent invention;

FIG. 3 is an isometric bottom view of a dielectric member which has beenpartially depopulated for clarity;

FIG. 4 is a partial cross-sectional side view of an embodiment of thepresent invention;

FIG. 5 is a partial isometric bottom view of the dielectric shown inFIG. 3 with conductive traces shown in phantom;

FIG. 6 is an isometric top view of the conductive trace shown in phantomin FIG. 5;

FIG. 7 is an isometric bottom view of the conductive trace shown inphantom in FIG. 5;

FIG. 8 is a top view of an alternative embodiment of the conductivetrace shown in FIGS. 6 and 7;

FIG. 9 is a top view of an alternative embodiment of the dielectricmember of the present invention with a cut-away showing conductivemembers; and

FIG. 10 is a partial cross-sectional side view of an alternativeembodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a cross-sectional side view of a conventional semi-conductorpackage 20 housed within a socket 30 and mounted to a printed circuitboard 10 by way of solder balls 40. Upon soldering socket 30 to printedcircuit board 10, interfaces 36 a and 36 b are created between thesolder ball 40 and contact 32 and between solder ball 40 and pcb 10,respectively. These interfaces 36 a, 36 b are subjected to stressesinduced by mismatches in the coefficients of thermal expansion (CTEs) ofthe socket and the printed circuit board. This mismatch in CTEs resultsin expansion and contraction due to thermal cycling occurring atdifferent rates in the socket and the printed circuit board. Thestresses experienced at interfaces 36 a, 36 b frequently result incracking or fracturing of those solder joints, resulting in mechanicaland electrical failure of at least part of the electrical assembly 8.

Another problem with the prior art configuration shown in FIG. 1 is thatduring the solder reflow operation when socket 30 is mounted ontoprinted circuit board 10, liquified solder balls 40 may, by capillaryaction, be drawn up along the sides of contacts 32 and away from printedcircuit board 10 resulting in a weakened interface 36 b. This problem,also known as solder-wicking, contributes to non-uniform solder jointsand reduced electrical performance.

FIG. 2 shows a partial cross-sectional side view of an embodiment of thepresent invention in which an electrical component 90 houses electricalcontact 92 having solder tail 94. Mounted beneath electrical component90 is a dielectric member 50 disposed between solder ball 40 and soldertail 94 making electrical contact therebetween by way of conductivemember 54. Electrical component 90 and dielectric member 50 arecollectively mounted to a printed circuit board 10 by way of solder ball40. Materials commonly used in manufacturing printed circuit boards haveCTE values within the range of 12 to 18 ppm/° C. Under the teachings ofthe present invention, it is desirable to approximate the CTE value ofdielectric member 50 to the CTE value of the printed circuit board 10.Approximating the CTE value of dielectric member 50 to the CTE of pcb10, reduces stresses induced in solder joint interfaces 36 a and 36 b,thereby minimizing the potential for solder joint fracture.

In the embodiment shown in FIG. 2, the effects of CTE mismatch areminimized between printed circuit board 10 and electronic component 90by selecting a dielectric member 50 comprised of a flexible film 52which houses a compliant conductive trace 54. Various flexible filmmaterials, such as polyimide films, may be used which offer a variety ofCTE values. For instance, a material which sells under the tradename“Kapton” performs well with most printed circuit boards. The flexiblefilm material 52 which is selected cooperates with the compliantconductive trace 54 to absorb the CTE mismatch which frequently existsbetween electrical component 90 and pcb 10. Electrical components havinginsulative housings molded from plastics may have CTE values up to 70ppm/° C. and sometimes higher. As such, although a dielectric member 50is implemented which has a CTE value approximating the CTE value of thepcb 10 a CTE mismatch frequently still exists between electricalcomponent 90 and dielectric member 50. However, because of thecompliancy of conductive trace 54 and because electrical contact 92 istypically not rigidly secured within electrical component 90, matchingthe CTE value of electrical component 90 with the CTE value ofdielectric 50 is much less of a concern than matching CTE values ofdielectric member 50 with that of pcb 10. In addition, as shown in FIG.2, solder tail 94 is received in through-hole 59 of dielectric member 50which inherently provides a more secure electrical connection than thesolder interfaces at 36 a and 36 b of solder ball 40.

Accordingly, in order to accommodate the solder ball 40 and solder tail94 arrangement shown in FIG. 2, dielectric member 50 is provided havinga conductive trace 54 held within flexible film 52 wherein both theflexible film 52 and the conductive trace 54 have a through-hole 59therethrough for receiving solder tail 94. Also, the bottom side ofdielectric 50 has a recess 58 in the flexible film 52 which receivessolder ball 40 allowing for electrical contact between solder ball 40and conductive trace 54. This recess 58 performs the added function ofserving as a solder reservoir for solder ball 40 upon solder reflow,thereby ensuring uniformity of solder joints and preventingsolder-wicking.

The arrangement shown in FIG. 2 lends itself well to applicationsrequiring a large array of electrical contacts within an electricalcomponent, such as that represented by reference number 90. Forinstance, microprocessors typically are packaged having largerectangular or square shaped arrays of pins that are received by sockethousings which are in turn mounted onto printed circuit boards. As such,a dielectric member having a preselected CTE substantially matched to aprinted circuit board would preferably be configured having an identicalarray as that required by the processor socket. FIG. 3 shows anisometric bottom view of dielectric member 50 having an array of throughholes 59 in flexible film 52 for receiving an array of solder tailsextending from a socket housing (not shown).

The dielectric member 50 shown in FIG. 3 is a particular embodimentwhich is further illustrated in FIG. 4. As shown in FIG. 4, sockethousing 30 houses an array of electrical contacts 32 (for the sake ofclarity, only two contacts are shown) having solder tails 34. Pins (notshown) from a semi-conductor package, such as a microprocessor, would bereceived in pin cavities 38, electrically mating with electricalcontacts 32. Below the socket housing 30 is dielectric member 50 (alsoshown in FIG. 3) which receives solder tails 34 in through-holes 59.Dielectric member 50 is comprised of a flexible film material 52 whichis preselected to have a CTE value which is sufficiently matched to theCTE value of printed circuit board 10 to effectively reduce undesirablestresses in solder joints 36 a and 36 b. Within the flexible filmmaterial 52 lie an array of conductive traces 54 which adhere toflexible film member 52 by way of adhesive layers of 56 a and 56 b shownon either side of conductive trace 54.

FIG. 5 shows a partial top isometric view of dielectric member 50 withconductive trace members 54 shown in phantom. FIGS. 6 and 7 are top andbottom isometric views, respectively, of the conductive trace member 54shown in phantom in FIG. 5. FIGS. 6 and 7 illustrate an hour glassshaped conductive trace 54 sandwiched between adhesive layers 56 a and56 b. Through-hole 59 is provided on one side of a necked-down portion57 for receiving a solder tail 34 as shown in FIG. 4. And, as shown inFIG. 7, the bottom adhesive layer 56 b has an opening 55 which exposessolder pad 53 for contacting solder ball 40 and defines a solderreservoir 58 as shown in FIG. 4. This solder reservoir 58 serves thepurposes of both locating the solder balls 40 onto the solder pads 53and containing the reflowed solder within the reservoir 58. Of course,other methods could be used to locate the solder balls 40 onto solderpads 53, such as providing holes through the solder pads which haverelatively narrower diameters than the diameters of the solder balls.

The hour glass shape of conductive trace 54 may be modified to simplifymanufacture of the dielectric member (for instance, a simple rectangularstrip of a compliant conductive metal could be used) while more complexpatterns may be employed to further enhance compliancy. One example of amodified conductive trace is shown in FIG. 8 wherein conductive member60 is provided with a solder pad 64 and a portion 63 having through hole62 connected by serpentine necked-down portion 66.

FIG. 9 shows another embodiment of the dielectric member of the presentinvention in which dielectric member 78 is constructed having arectangular array of conductive traces 79 running lengthwise with thedielectric member. As suggested by FIG. 9, numerous shapes may beemployed for the dielectric member in order to meet the requirements ofthe particular application. Also, the conductive traces may be arrangedin various fashions within the dielectric member. Thus, the dielectricmember may be easily adapted to meet the CTE requirements of countlesselectronics applications. And, just as the parameters of the dielectricmay be adjusted, so too may the conductive traces be modified tomaximize the performance of the dielectric member. For instance, theconductive metal used as the conductive trace may be selected based onthe CTE value of the particular metal. For example, a conductive tracemade of copper would typically have a CTE of about 20 ppm/° C., butother conductive materials having higher or lower CTE values could beused to further “tune” the system.

Another embodiment of the present invention is shown in FIG. 10 in whichdielectric member 80 has an alternative configuration. Similar to thedesign in FIG. 4, socket housing 70 houses an array of electricalcontacts 74 having solder tails 76. This socket housing 70 receives anarray of pins 22 which extend from a semi-conductor package 20 such as amicroprocessor and which electrically mate with contacts 74. Disposedbeneath socket housing 70 is dielectric member 80 which may bemanufactured from flexible film having CTE values which approximate CTEvalues of printed circuit board 10 or, as shown, may be constructed of aprinted circuit board material. This alternative provides the addedadvantage of being able to identically match the CTE value of thedielectric member 80 with the CTE value of the printed circuit board 10by selecting identical materials. In this embodiment, dielectric member80 is provided with solder pads 86 a and 86 b disposed on either side ofthe printed circuit board material 82 and electrically connected byplated through-hole 84. Solder tail 76 of electrical contact 74 is thendisposed within the plated through-hole 84 for electrically contactingsolder ball 72 which is affixed to solder pad 86 b. Although in thisconfiguration no solder reservoir is created, occurrences ofsolder-wicking are still reduced by offsetting the through-hole 84 andsolder ball 72 and by providing a more inhibitive barrier through theuse of a dielectric member 80.

It should be apparent from the foregoing description that the presentinvention provides an effective way of minimizing the negative effectsof CTE mismatch at electrical interfaces of separate electricalcomponents. And, although particular reference has been made tomicroprocessor sockets mounted onto printed circuit boards, it should beclear that various electrical components would benefit from the solutionto CTE mismatch set forth in the present invention. That is, electricalinterfaces that exist between electrical components having different CTEvalues, may be subjected to stresses induced by thermal expansion andcontraction. The use of a dielectric member constructed of a materialhaving a desired CTE that approximates the CTE of at least one of theelectrical components would serve to absorb a CTE mismatch at theinterface, thereby reducing the potential for electrical failure.

It should also be apparent that the conductive traces or solder padsreferred to throughout the description could be manufactured fromvarious conductive materials and arranged in a variety of patterns tosuit the particular application. For instance, copper traces could beused to common selected pins of a semiconductor package, creating ineffect a programmable dielectric member.

The dielectric member of the present invention and many of its attendantadvantages will be understood from the foregoing description. It isapparent that changes may be made in the form, construction, andarrangement of parts thereof without departing from the spirit of theinvention, or sacrificing all of its material advantages. Thus, whileseveral embodiments of the invention have been disclosed, it is to beunderstood that the invention is not strictly limited to thoseembodiments but may be otherwise variously embodied and practiced withinthe scope of the appended claims.

We claim:
 1. A socket for receiving a semiconductor package, comprising:a housing having a plurality of electrical contacts; a flexible filmdielectric having a top surface, a bottom-surface and a conductive traceprovided there between, a plurality of first conductive sites providedon the conductive trace and accessible via apertures in the top surfaceof the flexible film dielectric and a plurality of second conductivesites provided on the conductive trace and being electricallyinterconnected to the first conductive sites; a solder receiving recessprovided in the bottom surface of the flexible film dielectric andextending to the conductive trace, the recess provided to act as asolder reservoir and to properly position respective solder devicesrelative to the second conductive sites; respective first conductivesites and respective second conductive sites are disposed directlybeneath respective electrical contacts; wherein respective ones of theplurality of electrical contacts are electrically connected to the firstconductive sites and the second conductive sites are connected to anelectrical component.
 2. The socket of claim 1, wherein the dielectrichas a coefficient of thermal expansion lower than the coefficient ofthermal expansion of the housing and greater than or equal to thecoefficient of thermal expansion of the printed circuit board.
 3. Thesocket of claim 1, wherein each first conductive site and each secondconductive site is formed on a compliant electrically conductive memberheld within the flexible film dielectric.
 4. The socket of claim 3,wherein each compliant electrically conductive member has a narrowedportion between the first and second conductive sites.
 5. The socket ofclaim 3, wherein the electrically conductive site has a through-hole forreceiving a solder tail of the electrical contact and the secondconductive site is a solder pad for connecting to a solder ball.
 6. Thesocket of claim 5, wherein the flexible film dielectric defines areservoir about the exposed second conductive site for containing solderupon reflow of the solder ball.
 7. The socket of claim 1, wherein theflexible film dielectric is a polyimide material.